1. Field of the Invention
The present invention relates to a fuel injection control device, which performs learning of an injection quantity of a diesel engine.
2. Description of Related Art
A pilot injection for injecting a small amount of fuel before a main injection is performed in order to reduce combustion noise and to inhibit generation of nitrogen oxides in a diesel engine. Since a command value of an injection quantity of the pilot injection is small, accuracy of the small-amount injection should be improved in order to sufficiently exert the effect of the pilot injection of reducing the combustion noise and of inhibiting the nitrogen oxides. Therefore, learning should be preferably performed in the software for detecting a deviation between the command injection quantity of the pilot injection and a quantity of the actually injected fuel (an actual injection quantity) and for correcting the deviation.
However, it is difficult to directly measure the actual injection quantity while a vehicle is traveling. Therefore, conventionally, a method of continuously sensing an air fuel ratio, a pressure in a cylinder or the like as a substitute for the actual injection quantity has been employed as disclosed in Unexamined Japanese Patent Application Publication No. H11-294227, for instance. In this method, the deviation of the injection quantity can be detected accurately, and meanwhile, the deviation can be detected in an entire range of an operating mode.
As an alternative, a method of correcting the deviation of the injection quantity based on a correction value used in feedback control of rotation speed such as ISC (idling rotation speed control) has been also employed, for instance, as disclosed in Unexamined Japanese Patent Application Publication No. 2002-295291. For instance, on the condition that the rotation speed of the engine is preliminarily found to coincide with desired idling rotation speed when the actual injection quantity is 5 mm3/st, the command injection quantity is gradually changed so that the rotation speed of the engine coincides with the idling rotation speed. The injection quantity at the time when the rotation speed sensed by the rotation speed sensor or the like coincides with the idling rotation speed can be regarded as 5 mm3/st, even though the injection quantity is not measured directly. In this method, as explained above, a generally used sensor such as the rotation speed sensor is used. Therefore, no additional device is required and an increase in the cost can be inhibited.
However, in the method disclosed in Unexamined Japanese Patent Application Publication No. H11-294227, the air fuel ratio sensor and the cylinder inner pressure sensor, which are not mounted in the vehicle generally, are necessary. Thus, the additional devices are required and the cost is increased.
In the method disclosed in Unexamined Japanese Patent Application Publication No. 2002-295291, a unique relationship between the engine rotation speed and the injection quantity is broken because of a variation among cylinders of the engine or a fluctuation of load factors applied to the engine such as exterior loads including an air conditioner. The learning is performed based on the relationship between the rotation speed and the command injection quantity balanced under the influence of the fluctuation of the load factors. Therefore, it is difficult to accurately perform the learning of the injection quantity.
For instance, the engine rotation speed increases as the injection quantity increases and decreases as the injection quantity decreases. In addition, the engine rotation speed also fluctuates in accordance with the state of the loads of the air conditioner, an alternator and the like, which are applied to the engine. Therefore, the actual injection quantity cannot be sensed accurately by merely monitoring the engine rotation speed.